The present invention relates to coated abrasives and particularly those made using a phenol/formaldehyde binder in either a maker or size coat.
A coated abrasive is conventionally made by application of layer of a binder resin, (called a maker coat), to a backing material followed by a layer of abrasive grits and then at least partial cure of the resin binder to anchor the grits to the backing. A size coat is then typically applied over the abrasive grits and the cured. The main function of the size coat is to improve the retention of the abrasive particles to the substrate. In some cases it is advantageous to apply over the size coat a further layer comprising a binder and a functional additive such as a grinding aid, a lubricant or an antistatic agent. The binder resin used in each layer is frequently the same to avoid compatibility problems potentially associated with the use of dissimilar resins.
The most commonly used binder resins, particularly where abrasive grits with hydrophilic surfaces such as alumina-based abrasives are used, are phenolic resins. These have many very significant virtues such as hardness, adaptability, water-dispersibility and relatively low cost. They do however have a disadvantage that can be a practical problem. Many manufacturers prefer to associate at least some of their products with a particular color which is added in the form of a dye or pigment added to the binder formulation for at least the size coat. With such a commercial strategy, it is important that the color remain essentially constant from one production run to the next and within the same run. Unfortunately however resole resins, which are base catalyzed, in the presence of oxygen undergo over time a partial oxidation to form quinone derivatives which have a reddish color. This interferes with the added color and, since the degree of interference depends on the age of the resole and the degree of oxidation it has experienced, the extent of the interference, and therefore the resultant shade of the product, will vary significantly.
One way in which this problem has been approached in the past is the incorporation of a second binder component, such as a melamine resin, that inhibits the oxidation. This however is not a perfect solution because it leads to mixture stability problems and a consequent reduction in shelf life of the binder blend. Alternatively it has been suggested that an ammonium salt, such as ammonium aluminum fluoride or ammonium hydrogen fluoride, be incorporated in the binder. This is the approach advocated in U.S. Pat. Nos. 5,110,320 and 5,232,468. Quite substantial amounts of the additive are required however with up to about 40% by weight of the total alkali metal catalyzed phenolic resin being taught. This too can lead to compatibility and shelf life problems.
A novel approach to this problem has now been devised which avoids the need to load up the binder resin formulation with additives. The novel approach uses a binder with the virtues of phenolic resin binders but with significantly enhanced color stability. As an added bonus, the strong odor of phenol which characterizes many coated abrasives operations using phenolic binders is significantly reduced.